Among the attributes of a material that determine its usage is its ability to be joined with the same or other materials. In many cases, it is indispensable to assemble individual parts or components together into an additive product unit with complex structures. Joining of materials can not only offer a chance to scale up the structure, and especially joining of dissimilar materials, but also allows for introducing multifunctional heterogeneity into a structure. In order to achieve a metallic bond between two metal parts, they need to be brought together into atomic contact and the surfaces must be clean. Establishing these requirements is challenging. Most metals oxidize and a rigid oxide layer readily forms, which acts as a diffusion barrier and renders metallic bonding difficult.
Bulk metallic glasses (BMGs), also known as bulk solidifying amorphous alloy compositions, are a class of amorphous metallic alloy materials that are regarded as prospective materials for a vast range of applications because of their superior properties such as high yield strength, large elastic strain limit, and high corrosion resistance. BMG forming compositions may be based on titanium, copper, iron, nickel, palladium, zirconium, gold, cerium, platinum, calcium, magnesium, among others and alone or in combination with each other.
BMGs are regarded as prospective materials for a vast range of applications because of their superior properties such as high yield strength, large elastic strain limit, and high corrosion resistance. A unique property of BMG is that they have a super-cooled liquid region (SCLR), which is a relative measure of the stability of the viscous liquid regime. The SCLR is defined by the temperature difference between the onset of crystallization, Tx, and the glass transition temperature, Tg, of the particular BMG alloy. These values can be conveniently determined by using standard calorimetric techniques such as DSC (Differential Scanning calorimetry) measurements under a certain heating rate (e.g., at 20° C./min).
Thermoplastic forming (TPF) of an amorphous metal alloy involves heating it into the SCLR and forming it under an applied pressure. The method is similar to the processing of thermoplastics, where the formability, which is inversely proportional to the viscosity, increases with increasing temperature. In contrast to thermoplastics however, the highly viscous amorphous metal alloy is metastable and eventually crystallizes. Because BMGs are amorphous in nature, they can relax upon heating into supercooled liquid region in which the metallic glasses are metastable and are able to flow like plastics, allowing the thermoplastic forming of metallic glasses. Furthermore, it is worthy to note the time window that can be utilized to process metallic glasses is limited at a certain temperature above the glass transition temperature (Tg) to guard against crystallization tendency.
Crystallization of the amorphous metal alloy must be avoided for several reasons. First, it degrades the mechanical properties of the amorphous metal alloy. From a processing standpoint, crystallization limits the processing time for hot-forming operation because the flow in crystalline materials is order of magnitude higher than in the liquid amorphous metal alloy. Crystallization kinetics for various amorphous metal alloys allows processing times between minutes and hours in the described viscosity range. This makes the thermoplastic forming method a finely tunable process that can be performed at convenient time scales, enabling the net-shaping of complicated geometries. Since similar processing pressures and temperatures are used in the processing of thermoplastics, techniques used for thermoplastics, including compression molding, extrusion, blow molding, and injection molding have also been suggested for processing amorphous metal alloys as described, for example, in J. Schroers, “Processing of Bulk Metallic Glass,” Adv. Mater. 22 (14), 1566-1597 (2010).
To form amorphous metal alloys using a thermoplastic forming process, the amorphous metal alloy must be in its amorphous state, which means that the feedstock must be processed so that the sample is cooled fast to avoid crystallization. During this step, the amorphous metal alloy is typically not formed into its final shape but is rather cast into simple geometries such as cylinders, plates, pellets, and powders. Thereafter, the amorphous metal alloy is hot formed by reheating the material into the supercooled liquid temperature region where the material is formed under isothermal conditions, such that the amorphous state relaxes into a highly viscous metastable liquid that can be formed under applied pressure. Under isothermal conditions, the formability of the amorphous metal alloy increases with increasing processing temperature. Thus, the highest isothermal formability can be achieved at the highest possible processing temperature, so long as crystallization can be avoided.
The ability of an amorphous metal alloy to be thermoplastically formed is described by its formability, a parameter which is directly related to the interplay between the temperature dependent viscosity and time for crystallization as described, for example, in J. Schroers, “On the Formability of Bulk Metallic Glass in its Supercooled Liquid State,” Acta Mater. 56 (3), 471-478 (2008); H. Kato, T. Wada, M. Hasegawa, J. Saida, A. Inoue and H. S. Chen, “Fragility and Thermal Stability of Pt- and Pd-based Bulk Metallic Glass Forming Liquids and their Correlation with Deformability,” Scripta Mater. 54 (12), 2023-2027 (2006); and E. B. Pitt, G. Kumar and J. Schroers, “Temperature Dependent Formability in Metallic Glasses,” J. Appl. Phys. 110 (4) (2011). Crystallization has to be avoided during TPF of an amorphous metal alloy since it degrades the amorphous metal alloy's properties and retards its formability. Therefore, the total time elapsed during TPF of the amorphous metal alloy must be shorter than the time to crystallization.
The fabrication of BMGs requires a cooling rate (1˜104 K/s) from the molten liquid alloy to achieve a fully amorphous microstructure. This high required cooling rate has been a significant challenge for joining BMGs to both; similar and dissimilar materials. The high cooling rate has also imposed a size limitation on the BMGs, which has been a key issue for broadening the industrial applications of BMGs. Furthermore, the current fabrication route of BMGs is limited to casting, which can merely produce small-scale and more disappointedly simply geometrical samples. It is therefore of paramount significance to explore convenient and reliable techniques of joining metallic glasses for to extend their structural applications.
Previous studies have attempted thermoplastic joining of BMGs in the supercooled liquid region. However, these studies either required very long diffusion bonding time or required complex experimental conditions, such as a high vacuum level, to avoid surface contamination and oxidation. Furthermore, the results indicated that the joint quality was still unsatisfactory, either because of the oxide film layer between the metallic glasses, which impedes the atomic diffusion, or due to the unavoidable crystallization or phase transformation over the interface after a long period of processing time.
U.S. Pat. No. 7,947,134 to Lohwongwatana et al. the subject matter of which is herein incorporated by reference in its entirety, describes a thermoplastic joining method for joining metals using a BMG as a solder. However, Lohwongwatana is not joining BMGs but is instead using BMG as a thermoplastic joining solder for joining other metals. The bonding of Lohwongwatana relies on surface mechanical interlock by wetting of BMG on metals and no strain applied to the to-be-joined metal pieces. Flux is needed to reduce oxides and impurities on the metal and bulk metallic glass surfaces. In addition, oxygen-inert BMGs and high vacuum (10−6 mbar) are required to minimize oxidation during thermoplastic wetting of BMG on the metal surface, which results in low bonding strength (<50 MPa).
U.S. Pat. Pub. No. 2012/0288728 to Hofmann et al. the subject matter of which is herein incorporated by reference in its entirety, describes a joining method for BMGs that uses rapid capacitive charge. However, Hofmann uses an electrical discharge that is applied to the to-be-joined materials to melt the joint area. Because of the use of electrical discharge, the method described by Hofmann can only be used for sequential joining; parallel joining required for an industrial joining of many areas is not achievable by this method. In addition, the process of Hofmann is difficult to control when joining interior parts in a complex BMG structures and there is no predictable joint strength.
U.S. Pat. Pub. No. 2010/0275655 to Kawamura et al. the subject matter of which is herein incorporated by reference in its entirety, describes a joining method for BMGs by electron beam welding. However, Kawamura relies on a high energy electron beam to melt BMGs to bond them. In addition, electron beam welding is a liquid fusion process that is expensive and that utilizes a complicated apparatus and processing parameters. Electron beam welding also has a broad heat affected zone, suffers from easy crystallization of BMGs, and an uncontrollable interface quality. Electron beam welding also is only possible for sequential joining and there is no predictable joint strength.
Laser welding has also been suggested for joining BMGs to other BMGs or dissimilar materials, as described, for example, in B. Li, Z. Y. Li, J. G. Xiong, L. Xing, D. Wang, Y. Li, “Laser welding of Zr45Cu48Al7 bulk glassy alloy,” J. Alloy. Compd. 413 (1-2), 118-121 (2006); J. H. Kim, C. Lee, D. M. Lee, J. H. Sun, S. Y. Shin, J. C. Bae, “Pulsed Nd:YAG laser welding of Cu54Ni6Zr22Ti18 bulk metallic glass,” Mater Sci Eng A 449-451, 872-875 (2007), and Y. Kawahito, T. Terajima, H. Kimura, T. Kuroda, K. Nakata, S. Katayama, A. Inoue, “High-power fiber laser welding and its application to metallic glass Zr55Al10Ni5Cu30,” Mater Sci Eng B 148, 105-109 (2008). However, laser welding is expensive, and requires the optimization of many processing parameters, requires high power input, melting and a broad heat affected zone. In addition, laser welding suffers from potential crystallization and has a tendency to oxidize in the vicinity of the joint. Laser welding is also only suitable for sequential joining and also has no predictable joint strength.
A chemical reactive layer process involves the use of a layer of chemical reactive material and by lighting this inter-medium layer, fuses two BMG layers. Thus, the chemical reactive layer process is a liquid fusion process that relies on a chemical reaction and is expensive. Examples can be found in A. J. Swiston Jr., T. C. Hufnagel, T. P. Weihs, “Joining bulk metallic glass using reactive multilayer foils,” Scripta Mater 48, 1575-1580 (2003). and A. J. Swiston Jr., E. Besnoin, A. Duckham, O. M. Knio, T. P. Weihs, T. C. Hufnagel, “Thermal and microstructural effects of welding metallic glasses by self-propagating reactive multilayer foils,” Acta Materialia 53, 3713-3719 (2005). Chemical reactive layers have a tendency to melt and suffer from potential local crystallization due to the high amount of heat released during the reaction. In addition, the use of chemical reactive layers also introduces a heterogeneous substance or contaminants into the interface. The chemical reactive layer process is only suitable for sequential joining and also has no predictable joint strength.
Diffusion bonding within the supercooled liquid region is also very impractical. Diffusion bonding involves a long-term diffusion process that is very slow (i.e., >10 minutes), has the potential for crystallization and requires a high vacuum. Diffusion bonding also involves no thermoplastic strain and exhibits no predictable joint strength. An example can be found in P. H. Kuo, S. H. Wang, P. K. Liaw, G. J. Fan, H. T. Tsang, D. C. Qiao, F. Jiang, “Bulk metallic glasses joining in a supercooled-liquid region,” Materials Chemistry and Physics, 120, 532-536 (2010).
Various other welding processes for joining BMGs have also been suggested, including friction stir welding, resistance spot welding, and spark welding.
Friction stir welding is a liquid fusion process and suffers from uncontrollable welding processing. In addition, friction stir welding requires an ultra-high mechanical load and has high speed friction and a broad heat affected zone with potential local crystallization. Friction stir welding is available only for sequential joining and exhibits no predictable joint strength. An example of friction welding can be found in C. H. Wong, C. H. Shek, “Friction welding of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass,” Scripta Materialia 49, 393-397 (2003).
Resistance spot welding is a joining method that involves introducing an electrical resistance layer between two BMGs to generate significant heat to fuse the two BMGs together, such as described in D. Makhanlall, G. Wang, Y. J. Huang, D. F. Liu, J. Shen, “Joining of Ti-based bulk metallic glasses using resistance spot welding technology,” Journal of Materials Processing Technology 212, 1790-1795 (2012). Resistance spot welding is also a liquid fusion process and has been found to exhibit significant melting in the fusion zone. In addition, the significant temperature rise induces crystallization and creates a large heat affected zone in the vicinity of the weld. Resistance spot welding also requires a super high welding current and is a difficult process to control. Resistance spot welding is also available only for sequential joining and exhibits no predictable joint strength.
Spark welding is another liquid fusion process that requires ultrahigh current and suffers from melting at the interface of the weld. In addition, spark welding also has a large heat affected zone in the vicinity of the weld. Spark welding suffers from potential embrittlement or crystallization of the weld interface. Spark welding is also available only for sequential joining and exhibits no predictable joint strength. An example can found in Y. Kawamura, Yasuhide Ohno, “Sparking welding of Zr55Al10Ni5Cu30 bulk metallic glasses,” Scripta Materialia 45, 127-132 (2001).
Thus, it can be seen that a number of joining methods for BMGs have been investigated, all of which suffer from various deficiencies that make their use less than ideal and it would be desirable to provide an improved method of joining a BMG to a similar or dissimilar material that overcomes the deficiencies of the prior art.
Essentially, in order to obtain good bonding between two BMGs, a considerable amount of physical surface contact between the two BMGs and without contaminations in between is necessary to allow atomic diffusion and metallic bonding in a period of time that is less than the crystallization time of the BMG.
The inventors of the present invention have investigated many different methods for joining BMGs at various temperatures. For example, the inventors tried joining BMGs at room temperature, and failed to bond two BMGs, even with the use of a very high pressure (1 GPa) close to yield point to compress the metallic glasses together and allowed diffusion against each other for several hours, and even conducted in vacuum. The inventors also tried joining BMGs at elevated temperatures slightly above the glass transition temperature, where the viscosity of metallic glasses is still very high, and failed to bond two BMGs together by holding them together (to allow diffusion bonding) without straining. It appeared that the oxide or contaminants on the surface of the BMGs always impeded physical contact between pristine materials from both sides.
Oxidation has being a long-standing issue during the thermoplastic joining of BMGs. As described above, prior studies on joining of BMGs were all carried out under complicated experimental conditions, including high vacuum, to circumvent oxide formation. Previous joining methods also relied mainly on long-range diffusion or melting of the interface. Since diffusion kinetic and crystallization kinetic are similar, crystallization or embrittlement was a significant issue. Melting of the interface engenders pronounced heating, however more often associated with a large heat affected zone. Consequently, crystallization or embrittlement of interface has also been of concern.
The inventors of the present invention were surprised to discover that when thermoplastically forming metallic glasses and straining the surface, that it was possible to achieve pristine material on the surface that facilitated joining. By “pristine” what is meant is that the surface lacks oxides and/or contaminants. In addition, the inventors also determined that the fraction of this pristine surface (which is otherwise extremely difficult and completely impractical to achieve) is directly proportional to the strain the material undergoes on the surface. As such, the process described herein is completely different from diffusion bonding, which requires long range diffusion.
Here, the time scale for joining is set by the time it takes for the pristine BMG to reach the surface and can be estimated to be on the order of seconds or less depending in part on, for example, the thickness of the oxide film, the size of the cracks, the wetting of the BMG on the oxide, and on the forming pressure and temperature for typical processing conditions.